93] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 93 



where change in hydrogen-ion concentration and low temperature over- 

 come the phototropic reaction and cause the fish to turn back. The fish 

 will even leap out of the water upon the dry land in an effort to follow the 

 light. 



15. This night activity is due to moonlight of a certain intensity and 

 it occurs only when the light penetrates into the water. 



16. As the season goes on, a marked decrease in the abundance of 

 Labidesthes along the shore is noted. 



17. Data are given which show that the species in coming into the 

 shallows is attempting to maintain a pH equilibrium of approximately 7.7. 



18. Further data show that the species migrate to the deep water in 

 the morning with the rising pH value of the water due to the photosyn- 

 thetic activity of the shallow water vegetation. 



19. Experimental data show that the species avoid and react to a drop 

 of 0.4°C in the temperature of the water. 



20. Further experimental data show that the fish avoid both increasing 

 acidity and increasing alkalinity, but that the aversion to a higher alka- 

 linity is less marked than to a lower alkalinity; that a pH of 8.2 is tolerated 

 without resistance, and it is noted that this is the normal pH for sea water. 



21. Experiments on the blood of the fish show that there is a difference 

 in the pH values of the blood of young and adults; that of the young is 

 pH 7.71; of the adult 7.83. 



22. The migrations of the fish are therefore correlated with the hydro- 

 gen-ion concentration of the blood: the young fish with the low pH values 

 remain in that portion of the lake having the lowest pH readings, as they 

 begin to mature and the pH of their blood changes, they begin to come 

 into water having a higher pH value. 



23. The food habits of 550 individuals of all ages were studied. The 

 young are plankton eaters; as they begin their shoreward migrations a 

 change in food habits occurs and insect food tends to replace the plankton 

 organisms; that the adults are very largely insectivorous during the sum- 

 mer, but tend to revert to the food habits of the immature during the 

 winter. 



24. Examination of 478 adults has shown the presence of but a single 

 annulus on the scales, from which, together with the decreasing abundance 

 of the adults in late summer, it must be concluded that the individuals 

 live for but fifteen to seventeen months. 



